In the construction of many buildings, a finished ceiling, which is referred to as a drop ceiling, is spaced below a structural floor panel that is constructed of concrete, for example. The drop ceiling supports light fixtures and other ceiling-mounted items, while the space between the ceiling and the structural floor from which it is suspended serves as a return-air-plenum for elements of heating and cooling systems as well as a convenient location for the installation of communications, computer and alarm system cables. It is not uncommon for these plenums to be continuous throughout the length and width of each floor.
When a fire occurs in an area between a floor and a drop ceiling thereabove, it may be contained by walls and other building elements which enclose that area. However, when and if the fire reaches the plenum, and if flammable material occupies the plenum, the fire can spread quickly through an entire story of the building and smoke can be conveyed through the plenum to adjacent areas. The fire could travel along the length of communications cables which are installed in the plenum and which comprise a plurality of conductors individually insulated with plastic material and enclosed in a jacket comprising a plastic material.
Because of the possibility of such flame spread and smoke evolution, particularly when aided by flammable insulation of cables, the 1975 edition of the National Electric Code (NEC) prohibited the use of electrical cables in plenums unless they were enclosed in metal conduits. Because rigid metal conduits are difficult to route in plenums congested with other items, a rearrangement of office telephones, which in some companies has almost become an annual event, is extremely expensive. However, the code permits certain exceptions to this cost prohibitive requirement. For example, flame retardant, low smoke producing cables without metallic conduit are permitted provided that such cables are tested and approved by an authority such as the well known Underwriters Laboratories.
Also, in buildings, cables are needed for use in riser shafts. Such cables should not be too heavy, otherwise, it may become difficult to pull long lengths over several stories. Acceptable riser cables also are governed by requirements set forth by the Underwriters Laboratories.
What is needed is a cable for use in buildings which is relatively inexpensive to manufacture, but which meets the NEC requirements for flame retardance and smoke evolution, and which has excellent mechanical properties, particularly mechanical flexibility.
In the marketplace, cable which comprises a core having a paper core wrap and enclosed in a relatively thick metallic shield is available, but it is relatively inflexible and somewhat difficult to maneuver in plenums. Moreover, care must be taken during installation to guard against possible electrical shock which may be caused by the metallic sheath of the above-described cable engaging exposed electrical service wires or equipment in a plenum. Also, while the above-described cable meets flame spread requirements of the code, the snugness with which the metallic shield encloses the conductors prevents a charring of the conductor insulation that could effectively seal off a portion of the cable about the flame and reduce the evolution of smoke. Fluoropolymer plastic material has been accepted as the covering material for plenum cable without the use of metal conduit, but it is relatively expensive and is difficult to process. Fire retardant polyvinyl chloride based materials are currently evolving as plenum grade materials, but typically they have much higher dielectric constants than fluoropolymers used as insulation materials.
One approach to the problems of flame spread and smoke evolution is to include a barrier layer in a cable. The prior art includes a cable having a barrier layer that is made of an inorganic, cellular material and that encloses the core, and a metallic barrier having longitudinal edge portions that form a seam. In order to be able to reflect radiant heat outwardly, an outwardly facing major surface of the metallic barrier has an emissivity in the range of about 0.039 to 0.057. The metallic barrier is covered with an inner tape comprising a thermosetting material having at least translucent optical clarity and having a relatively low thermal diffusivity which in a preferred embodiment is in the range of about 0.0008 to 0.001 cm.sup.2 /sec., and a second tape which is identical to the inner tape. The inner and the outer tapes are wrapped about the metallic barrier to form overlapped seams which are sealed. Such a cable is disclosed in U.S. Pat. No. 4,284,842 which issued on Aug. 18, 1981 in the names of C. J. Arroyo, N. J. Cogelia, and R. J. Darsey.
Another disclosure of a barrier material which includes ethylene copolymers with enhanced fire resistant properties appears in European Patent Application 0 248,404 which was filed Jun. 2, 1987 and which is incorporated by reference hereinto. The composition includes an ethylene copolymer, a mixture of aluminum trihydrate and calcium carbonate or calcium-magnesium carbonate or both and a phosphate ester. Optimally, the composition may include a borosilicate glass. The phosphate ester increases the flexibility of the composition. As the material decompresses or burns, the aluminum and calcium constituents form a ceramic ash that has a cell structure. As the ash builds up, the ash becomes a thermal insulator. The borosilicate glass acts to harden the ash at lower temperatures than those that normally activate the Ca-Al complex.
One of the problems in providing a superior flame retardant communications cable is that of meeting properties which run counter to each other. A desired property for an insulation material is a relatively low dielectric constant. This becomes important in today's world in which higher transmission frequencies and bit rates are demanded by customers. However, insulating materials of the prior art, such as polyethylene, for example, which exhibit a relatively low dielectric constant, do not have suitable resistance to flame spread and smoke evolution.
A conventional solution to the use of organic resins which are desirable from the standpoint of electrical properties is to include additives which delay establishment of a fire. Such a solution has limitations. Flame retardant additives only delay the onset of a fire and lose their effectiveness once the temperature exceeds a critical threshold. Also, the use of halogenated and phosphorous flame retardants may cause evolution of smoke which includes corrosive gases. Further, the addition of low molecular weight halogenated or phosphorous constituents may lead to a plasticizing effect on the mechanical behavior of the resultant material. Mineral fillers added to provide flame retardancy may compromise the mechanical properties of the resultant material and most certainly will compromise the electrical properties. Typically, the better the fire resistance of typically used plastic cable materials, the higher the dielectric constant because of the required inclusion of additive systems. Presently, there appears to be no widely accepted way in which to use polyolefin insulation with a low percent of additives in a high fire environment.
Although inexpensive halogenated plastic materials are fire retardant, they do not have the dielectric properties which are desired. For example, polyvinyl chloride (PVC) materials may be used for plenum cables, but they have a relatively high dielectric constant.
An additional desired property of both insulating and jacketing compositions is the absence of intrinsic or added halogens which may be based upon the desire to reduce corrosive combustion gases. For example, in optical fiber cables, the concern shifts away from the dielectric constant to corrosivity. In such cables, inasmuch as the dielectric constant is not of concern, non-halogenated materials may be sought after to avoid the problem of corrosivity.
What is desired and what seemingly is not provided in the prior art is a cable in which transmission media are covered with a plastic material dictated by electrical or other properties such as, for example, one which exhibits a desirably low dielectric constant and which cable also exhibits suitable resistance to flame spread and smoke generation. The sought after cable desirably is reasonable in cost and relatively easy to process.